As lubricants for the production of seamless tubes, so-called oily-type lubricants comprising graphite dispersed in fuel oils and so-called water dispersion-type lubricants comprising graphite dispersed in water have generally been used.
The use of an oily-type lubricant produces a great amount of soot that contaminates working environments and is a fire hazard since the oily-type lubricant contains fuel oils. In view of the above, the use of the water dispersion-type lubricant with no such disadvantages has been preferred in recent years. However, since the water dispersion type lubricants generally have poor adhesion to the surface of mandrel bars and low resistance to water, films formed with the water dispersion type lubricants have the disadvantage of being liable to detachment during transportation of the mandrel bar.
The invention disclosed in Japanese Patent Laid-Open No. 185393/1982 by Nihon Kokan K.K. and Yushiro Kagaku Kogyo K.K. and the invention disclosed in U.S. Pat. No. 4,001,125 by A. R. Newton are directed to the improvement of such water dispersion-type lubricants as described above. Although many improvements have been attained by these inventions, they are not yet satisfactory as will be explained hereinafter.
The temperature of the mandrel bar when coated with the lubricant varies depending on the processing conditions and generally varies over a wide range of from 60.degree. to 450.degree. C. The lubricant comprising graphite and gilsonite which is disclosed in U.S. Pat. No. 4,001,125 is poor in adhesion to the mandrel bar and in its water-resistance property at some temperatures. In fact, if the temperature at the surface of the mandrel bar is relatively low, for example less than 100.degree. C., the lubricant cannot provide a sufficient lubricating effect.
The invention disclosed in Japanese Laid-Open No. 185393/1982 concerns a lubricant having a glass transition point from 45.degree. to 130.degree. C. and preferably comprising from 5 to 15% by weight of gilsonite powder and from 70 to 90% by weight of graphite dispersed in water. However, this lubricant is less adhesive to the mandrel bar if the temperature of the surface of the mandrel bar is higher than 250.degree. C., and therefore it cannot provide sufficient lubrication. This Japanese reference also discloses lubricants for water dispersion containing 20 weight % resin, 15 or 20 weight % gilsonite powder and 60 or 65 weight % graphite. However, these lubricants do not form homogeneous films at temperatures of from 400.degree. to 450.degree. C. nor do they form sufficiently thick films at these higher temperatures.